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Prepare a 10’ talk for Friday March 3 on plant defense responses or describe interactions between plants& pathogens, pests or symbionts Plant defense responses Some possible pathogens • Hypersensitive response • Agrobacterium tumefaciens • Systemic acquired • Agrobacterium rhizogenes resistance • Pseudomonas syringeae • Innate immunity • Pseudomonas aeruginosa • Phytoalexin synthesis • Viroids • Defensins and other • DNA viruses proteins • RNA viruses • Oxidative burst • Fungi Some possible pests • Oomycetes • Nematodes Some possible symbionts • Rootworms • N-fixing bacteria • Aphids • N-fixing cyanobacteria • Thrips • Endomycorrhizae • Gypsy moths • Ectomycorrhizae • hemlock woolly adelgid Photosynthesis 2 sets of rxns in separate parts of chloroplast Light-independent (dark) reactions Overall Reaction: 3 CO2 + 3 RuBP + 9 ATP + 6 NADPH = 3 RuBP + 9 ADP + 9 Pi + 6 NADP+ + 1 Glyceraldehyde 3-P Light-independent (dark) reactions 1) fixing CO2 2) reversing glycolysis 3) regenerating RuBP fixing CO2 1) RuBP binds CO2 fixing CO2 1) CO2 is bound to RuBP 2) rapidly splits into two 3-Phosphoglycerate • therefore called C3 photosynthesis •detected by immediately killing cells fed 14CO2 fixing CO2 1) CO2 is bound to RuBP 2) rapidly splits into two 3-Phosphoglycerate 3) catalyzed by Rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) the most important & abundant protein on earth •Lousy Km •Rotten Vmax! •Makes lots of mistakes! Reversing glycolysis converts 3-Phosphoglycerate to G3P consumes 1 ATP & 1 NADPH Reversing glycolysis G3P has 2 possible fates 1) 1 in 6 becomes (CH2O)n Reversing glycolysis G3P has 2 possible fates 1) 1 in 6 becomes (CH2O)n 2) 5 in 6 regenerate RuBP Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi mechanism to regulate PS Regenerating RuBP G3P has 2 possible fates 5 in 6 regenerate RuBP necessary to keep cycle going Regenerating RuBP Basic problem: converting a 3C to a 5C compound feed in five 3C sugars, recover three 5C sugars Regenerating RuBP Basic problem: converting a 3C to a 5C compound must assemble intermediates that can be broken into 5 C sugars after adding 3C subunit Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C Uses 1 ATP/RuBP Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping Use 12 NADPH and 18 ATP to make one 6C sugar Regulating the Calvin Cycle Rubisco is main rate-limiting step Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase: Rubisco must be carbamylated & bind Mg2+ to be active! Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase : uses ATP to activate rubisco Regulating the Calvin Cycle Rubisco is main rate-limiting step • Rubisco must be carbamylated & bind Mg2+ to be active! • RuBP binds & inactivates uncarbamylated rubisco • Rubisco activase removes this RuBP Regulating the Calvin Cycle Rubisco is main rate-limiting step • Rubisco must be carbamylated & bind Mg2+ to be active! • RuBP binds & inactivates uncarbamylated rubisco • Rubisco activase removes this RuBP • In the dark many species phosphorylate carboxyarabinotol to form carboxyarabinitol 1phosphate which binds the rubisco active site Regulating the Calvin Cycle Rubisco activase removes this RuBP • In the dark many species phosphorylate carboxyarabinotol to form carboxyarabinitol 1phosphate which binds the rubisco active site • Rubisco activase also removes CA1P in the light • CA1P phosphatase then removes the PO4 Regulating the Calvin Cycle Availability of CO2 Demand is set by mesophyll, stomata control supply Ci is usually much lower than Ca A vs Ci plots tattle on the Calvin cycle Regulating the Calvin Cycle A vs Ci plots tattle on the Calvin cycle • In linear phase rubisco is limiting • When curves RuBP or Pi regeneration is limiting Regulating the Calvin Cycle Currently Rubisco usually limits C3 plants Will increase plant growth until hit new limiting factor Regulating the Calvin Cycle Currently Rubisco usually limits C3 plants Will increase plant growth until hit new limiting factor Free-Air CO2 Enrichment Experiments show initial gains, but taper off w/in a few years Now are limited by nutrients or water Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma a) pH: rubisco is most active at pH > 8 (in dark pH is ~7.2) Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma a) pH: rubisco is most active at pH > 8 b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma a) pH: rubisco is most active at pH > 8 b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark Mg2+ moves from thylakoid lumen to stroma to maintain charge neutrality Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma a) pH: rubisco is most active at pH > 8 b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark c) CO2 is an allosteric activator of rubisco that only binds at high pH and high [Mg2+] also: stomates open in the light Regulating the Calvin Cycle Rubisco is main rate-limiting step indirectly regulated by light 2 ways 1) Rubisco activase 2) Light-induced changes in stroma Several other Calvin cycle enzymes (e.g.Fructose-1,6bisphosphatase) are also activated by high pH & [Mg2+] Regulating the Calvin Cycle Several Calvin cycle enzymes (e.g.Fructose-1,6bisphosphatase) are also regulated by thioredoxin contain disulfide bonds which get oxidized in the dark Regulating the Calvin Cycle Several Calvin cycle enzymes (e.g.Fructose-1,6bisphosphatase) are also regulated by thioredoxin contain disulfide bonds which get oxidized in the dark in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme SH SH light 2Fdox PSI + PSII 2e- 2Fdred reduced thioredoxin S-S oxidized thioredoxin S-S oxidized enzyme (inactive) SH SH reduced enzyme (active) Regulating the Calvin Cycle Several Calvin cycle enzymes (e.g.Fructose-1,6bisphosphatase) are also regulated by thioredoxin contain disulfide bonds which get oxidized in the dark in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme How light reactions talk to the Calvin cycle SH SH light 2Fdox PSI + PSII 2e- 2Fdred reduced thioredoxin S-S oxidized thioredoxin S-S oxidized enzyme (inactive) SH SH reduced enzyme (active) PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + phosphoglycolate PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate Releases CO2 without making ATP or NADH PHOTORESPIRATION Releases CO2 without making ATP or NADH Called photorespiration : undoes photosynthesis PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon Both rxns occur at same active site PHOTORESPIRATION C3 plants can lose 25%-50% of their fixed carbon phosphoglycolate is converted to glycolate : poison! Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine produce H2O2 Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 Why photorespiration loses CO2 Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 5) serine is returned to peroxisome Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes 2) peroxisomes convert it to glycine 3) glycine is sent to mitochondria 4) mitochondria convert 2 glycine to 1 serine + 1 CO2 5) serine is returned to peroxisome 6) peroxisome converts it to glycerate & returns it to chloroplast Detoxifying Glycolate Why peroxisomes are next to cp and mito in C3 plants Mitochondrion C4 and CAM photosynthesis Rubisco can use O2 as substrate instead of CO2 [CO2] is 1/600 [O2] _-> usually discriminate well C4 and CAM photosynthesis Rubisco can use O2 as substrate instead of CO2 [CO2] is 1/600 [O2] Photorespiration increases with temperature C4 and CAM photosynthesis Rubisco can use O2 as substrate instead of CO2 [CO2] is 1/600 [O2] Photorespiration increases with temperature Solution: increase [CO2] at rubisco C4 and CAM photosynthesis Solution: increase [CO2] at rubisco C4 & CAM = adaptations that reduce PR & water loss C4 and CAM photosynthesis Adaptations that reduce PR & water loss Both fix CO2 with a different enzyme C4 and CAM photosynthesis Adaptations that reduce PR & water loss Both fix CO2 with a different enzyme later release CO2 to be fixed by rubisco use energy to increase [CO2] at rubisco C4 and CAM photosynthesis Adaptations that reduce PR & water loss Both fix CO2 with a different enzyme later release CO2 to be fixed by rubisco use energy to increase [CO2] at rubisco C4 isolates rubisco spatially (e.g. corn) C4 and CAM photosynthesis Adaptations that reduce PR & water loss Both fix CO2 with a different enzyme later release CO2 to be fixed by rubisco use energy to increase [CO2] at rubisco C4 isolates rubisco spatially (e.g. corn) CAM isolates rubisco temporally (e.g. cacti)
